1. Field of the Invention
The present invention relates to a process for producing organopolysiloxanes.
2. Description of the Prior Art
Heretofore, as a process of producing an organopolysiloxane, is known a process in which halosilanes such as organochlorosilanes and water are mixed, thereby these being hydrolyzed and condensed. This process is advantageous in cost, but where starting materials mainly contain three functional silanes, the reaction solution itself may form a gel or organopolysiloxane microgels may be formed in the reaction solution, so that processability is lowered.
As a process of production capable of controlling the gelation or formation of the microgel, a process is known in which alkoxysilanes which exhibit a lower hydrolysis rate than the halosilanes are hydrolyzed and condensed using an acid or alkaline catalyst. According to this process, not only the hydrolysis rate of the alkoxysilanes but also the rate of condensation of silanol groups are low. Consequently, the resulting organopolysiloxane contains an increased unreacted silanol groups, and has a broad molecular weight distribution. That is, for example, an organopolysiloxane having a weight average molecular weight of about 5,000 will have a polydispersion degree (ratio of weight average molecular weight (Mw) to number average molecular weight (Mw)) of 3 or more. The organopolysiloxane having self-condensable silanol groups produced by the process can be used as a starting material for heat-curable silicone resin compositions in which silanol groups act as functional groups, but it is unsuitable as a raw material for moisture-curable silicone resin compositions in which hydrolyzable groups such as alkoxyl groups act as a crosslinking functional group because it makes the shelf stability of the composition poor. Since the resulting organopolysiloxane has a broad distribution of molecular weight, it is liable to form a gel due to a high molecular weight organopolysiloxane contained, and curable silicone compositions prepared therefrom have a poor drying property and curability due to low molecular weight organopolysiloxanes contained. Furthermore, where the low molecular weight organopolysiloxanes are volatile, there is a fear of catching fire. The low molecular weight components also have disadvantage of contaminating surroundings.